1. Field of the Invention
The present invention relates to a light beam optical scanning system, and more particularly to a light beam optical scanning system to be employed in a laser printer, a facsimile or the like for imaging a light beam which carries image data.
2. Description of Related Art
A light beam optical scanning system employed in a laser printer or a facsimile generally comprises a laser diode, a scanner such as a polygonal mirror and a galvano mirror, and an f.theta. lens. The scanner deflects a light beam emitted from the laser diode at a constant angular velocity, and with no f.theta. system, when the light beam is scanned on a light receiving surface, the scanning speed varies between the center portion and the edge portions. The f.theta. lens is provided in order to adjust the scanning speed, that is, to correct distortion. The f.theta. lens is composed of various concave lenses and convex lenses. Therefore design of the f.theta. lens is very complex, and many surfaces must be subjected to polishing, which is not expected to be improved in accuracy. Such an f.theta. lens is costly. Additionally, the lenses must be made of a material which has a high light transmittance.
Japanese Patent Laid Open Publication No. 1-200219 suggested, in order to downsize the optical system, that a spherical mirror, which can be produced easily with a high accuracy, be used instead of the f.theta. lens, which is costly and is produced under severe constraint.
Further, as an improvement of this type of optical system which employs a mirror as an f.theta. system, the applicants suggested that a toroidal mirror be used (refer to Japanese Patent Laid Open Publication No. 4-194814). In the optical system, only one reflective surface of the toroidal mirror functions to correct distortion (as an f.theta. system), to correct curvature of field in the main scanning direction, to correct curvature of field in the sub scanning direction and to correct an error caused by misalignment of the reflective facets of a polygonal mirror. However, in order to obtain sufficient effect of the f.theta. mirror in these functions, the f.theta. mirror must be disposed substantially in the middle of the optical path of the scanning system, that is, the f.theta. mirror must be disposed such that the distance between the polygonal mirror and the f.theta. mirror will be substantially equal to the distance between the f.theta. mirror and the light receiving surface. More specifically, if the ratio of these distances gets out of 1:1, either the function of correcting distortion or the function of correcting curvature of field in the main scanning direction will be weaker. The components of the optical system must be disposed so as to meet the condition, and the components has very little freedom of positioning. Additionally, in connection with the space for the optical system, there may be a case wherein many mirrors have to be provided to make the optical path meet the condition.